1. The Yellow "Petals" and the Brown Center Are Actually Different Flowers
The parts that look like yellow petals are actually individual flowers called "ray florets." They surround the large head in a circle. The brown center is packed with even smaller "disk florets." It is explained that only these disk florets produce seeds [1]. This means a sunflower is a cluster of many flowers looking like one big flower. Such a cluster is called a "capitulum" or head inflorescence. Each grain in the center was once a separate flower.
What we call "seeds" are technically fruits (achenes) in botany. The sunflower originates from central and western North America. It is used for food, oil, soap, and paint ingredients [1].
2. Which Numbers Appear When You Count a Sunflower's Seed Spirals?
If you look closely at the center of a sunflower, the seeds are arranged in spirals. There are two types of spirals: clockwise and counter-clockwise. When counting the number of spirals in each direction, for example 34 and 55, they often turn out to be adjacent Fibonacci numbers [2]. Fibonacci numbers are a sequence where each number is the sum of the two preceding ones: "1, 1, 2, 3, 5, 8, 13, 21, 34, 55..."
There was an attempt to verify this observation through citizen power. To mark the 100th anniversary of the birth of mathematician Alan Turing (born in 1912), the Science Museum Group and others in the UK launched "Turing's Sunflowers," calling on people in various places to count sunflower spirals [2]. It is noted that Turing was interested in how plants show Fibonacci arrangements [2].
3. How Many of the 657 Sunflowers in the Citizen Experiment Had Fibonacci Numbers?
The results of the citizen-participation experiment were reported in a Royal Society academic journal in 2016 [3]. They examined data from 657 sunflowers. Of the 768 spiral counts included in the group considered highly reliable, 565 were Fibonacci numbers. This is about 74% (calculated as 565 ÷ 768).
Of the remaining cases, roughly 20% did not have a Fibonacci structure. It was reported that numbers "one less than a Fibonacci number" appeared significantly more often [3]. From this result, we can see that it is not true that sunflower seeds always arrange in Fibonacci numbers. The view from this survey is that many become Fibonacci numbers, but not all.
4. Why Can Sunflower Seeds Be Packed Without Gaps?
A reason often introduced for why seeds pack without gaps is the "golden angle." The golden angle is the smaller angle when 360 degrees is divided by the golden ratio, which is about 137.5 degrees [4]. It is explained that if you arrange seeds starting from the center, shifting each one by about 137.5 degrees, gaps are less likely to form between them.
Why do they arrange this way? Plus Magazine suggests that as the plant grows, this arrangement may have developed to pack "as many seeds as possible into the flower head" [2]. However, this is a hypothesis about evolution. As noted in the previous section, about 137.5 degrees is the value of an idealized model. Real sunflowers may deviate slightly from this.
5. Why Do Sunflowers Face East After They Bloom?
Young sunflowers are known to change direction during the day, tracking the sun, and return to face east during the night. However, once the flower opens, it stops moving and remains facing east [5]. This movement involves the body clock (circadian rhythm). It is reported that the direction changes by alternately adjusting the growth speed of the east and west sides of the stem [5].
Facing east had an advantage. East-facing flowers warmed up earlier in the morning, and it is said that five times more pollen-carrying insects visited them compared to west-facing flowers. When west-facing flowers were heated with a heater, insects returned, leading to the belief that temperature is involved [5]. The figure of "five times" is based on reported commentary. Please read this as the result of one study.
6. How Can You Count the Spirals in a Sunflower's Center Yourself?
If there is a sunflower nearby, try counting the spirals in the center yourself. If you don't have a real one, you can try with enlarged photos or pictures in encyclopedias. When counting, choose one spiral going from the center outward, and mark it as you count to reduce errors.
Count both clockwise and counter-clockwise spirals to see if they are adjacent Fibonacci numbers. Sometimes they are not. Since about 20% were not in the citizen experiment, getting different numbers is not strange.
Please handle the flowers gently. Flowers in gardens or parks are for others to see too.